Fuel cells are used in many applications. They are for example used as a source of energy in electric motor vehicles, or for recharging batteries, for example for recharging batteries of portable devices (telephones for example).
The electric energy produced by a fuel cell cannot usually be used directly. Specifically the cell does not supply a constant voltage. The voltage produced is usually weaker than necessary for the application, the number of cells of the fuel cell usually being optimized according to technical and cost criteria. Finally, the output voltage varies with the current.
Power supplies with non-isolated DC DC splitting are therefore used to regulate and bring the voltage supplied by a fuel cell to a required level.
These power supplies, also called converters or choppers, allow conversion from one continuous voltage to another continuous voltage over a voltage range from a few volts to a few thousand volts. More generally, they make it possible to convert one continuous voltage to another continuous voltage which may be higher or lower and which may be of the same polarity or of inverse polarity, depending on the topology of the power supply.
A power supply with DC DC splitting is an electric circuit that is usually tripolar with an input terminal, an output terminal and a common terminal. This electric circuit comprises at least one switch and one diode and one energy storage element, typically an inductor. The input voltage is applied between the input terminal and the common. An output capacitor is connected between the output terminal and the common. The transfer of energy from the input to the output is carried out by means of the energy storage element which stores the energy and then restores it at the rate of the switching of the switch to the open state and the closed state. The voltage is regulated by the conduction time (closed state) of the splitting switch.
Depending on the power range and on the gain sought for an application, various well known power supply topologies are proposed. These are the voltage step-up, called “boost”, voltage step-down, “buck”, voltage inverter and step-up/step-down, “buck-boost” topologies, “Cuk” topologies from the name of its inventor or voltage step-up/step-down or SEPIC (“Single ended primary inductor converter”) topologies.
The switch S is usually produced by a field effect transistor. That is why reference is made without distinction to a switch in the open state, or off state, and the closed state or on state. Typically in the range of input and output voltages from a few volts to several thousands of volts, use is preferably made of a transistor of the IGBT (Isolated Gate Bipolar Transistor) type capable of withstanding high voltages at its terminals. This technological solution makes it possible to ensure the reliability of the converter while minimizing the cost of the components.
FIG. 1a therefore illustrates a voltage step-up converter BC (boost). It is a tripole with a star topology (as for the buck or buck/boost converters): a switch S, an inductor L and a diode D each form one branch of the tripole. The branches all start from a common node A, and their termination forms one of the three terminals of the tripole.
In a converter of the step-up type, the switch S is connected between the node A and the common terminal B3. The diode has its anode connected to the node A, and its cathode connected to the output terminal B2. The inductor L is connected between the input terminal B1 and the node A.
The switch is controlled usually by a pulse signal with constant frequency which alternately places it in an open state and a closed state.
The two operating phases of such a converter, which correspond to the two states, closed and open, of the switch S, are as follows:                when the switch S is closed: the inductor L is in parallel on the input voltage source and the current increases in the inductor. This is the energy storage phase. The diode D is then disabled. The equivalent wiring diagram is illustrated in FIG. 1b.         when the switch S is open, the inductor L is in series with the input voltage source Ue. The current passes through the inductor L and the diode D and the output capacitor Cs is charged. This is the energy transfer phase. The equivalent wiring diagram is illustrated in FIG. 1c.         
The voltage at the terminals of the output capacitor Cs becomes higher than the input voltage. The output voltage level depends in practice on the durations of the open and closed times of the switch. If the splitting power supply works at a constant frequency f and in continuous conduction mode (that is to say that the current passing through the inductor is never cancelled out), the output voltage Us is equal to α*Ue, where a is the duty factor between the closing time of the switch and the complete period of the cycle (1/f=t).
In a known manner, splitting power supplies have the drawback of causing a ripple of the current in the output capacitor, and at input. The amplitude of the ripple is moreover one of the criteria for measuring the quality of such a power supply.
Certain electric energy sources such as the fuel cell for example do not withstand such a ripple of current, which has the effect of reducing its service life.
To solve this problem, it is known practice to use a splitting power supply with interlaced cells. Each cell is a converter. The concept of interlacing arises from the fact that the cells conduct in turn to an output capacitor.
FIG. 2 illustrates such a splitting power supply with interlaced cells in the voltage step-up (boost) converter topology of FIG. 1a. More particularly it illustrates a first exemplary embodiment in which the cells are all connected to the same output capacitor Cs.
In the example, the power supply comprises n=3 identical cells BC1, BC2, BC3 (L, S, D) in parallel: their terminals B1 are connected together; their terminals B2 are connected together; their terminals B3 are connected together. The power supply comprises a single output capacitor CS connected between the output terminals B2 and common terminals B3 of each cell. The input voltage Ue is applied between the input terminals B1 and common terminals B3 of each cell.
The n switches are each controlled as indicated above with FIG. 1a, with a time shift of fixed duration, corresponding to a phase shift between each cell of 2π/nf. The frequency of the currents and voltages seen by the load is therefore n times greater than that which is obtained with a single cell. The ripples at the input and in the output capacitor are reduced.
In the exemplary embodiment of FIG. 2, there is a single output capacitor Cs for the n interlaced cells. It is therefore a high-volume capacitor. In each cell, there is a wiring inductance Lw of the switch S, diode D, capacitor Cs loop. Since the capacitor is of high volume, the connections necessary for producing each loop are long. For these reasons the wiring inductances Lw are high and induce considerable losses. The efficiency of the power supply is degraded.
Moreover, provision is usually made to place the switches and diodes at the surface of a heat sink or of a water plate in order to allow the evacuation of the heat losses dissipated by these components. For the evacuation to be effective, it is necessary to spread the switches and diodes over the surface of the heat sink or of the water plate so as to ensure a certain distance between them. In these conditions, with a single output capacitor, common to the n cells, the distances between the switches and the diodes are increased which has the effect of also increasing the wiring inductances.
One way of solving this problem is to provide an output capacitor Cs1, Cs2, Cs3 for each cell BC1, BC2, BC3 as illustrated in FIG. 3. The wiring inductances Lw of the loops are then reduced by the closeness of the components of each cell to the associated capacitor. The various switches and diodes can be better spread on the surface of the cooling device without degrading these inductances Lw, allowing an optimized evacuation of the heat losses of these components. Each capacitor is placed close to the associated diode and switch.
But this produces additional wiring inductances L′w between the capacitors (FIG. 3). These wiring inductances have the drawback of opposing the interchanges of currents between the capacitors and of limiting the effectiveness of the interlacing: the effective currents in each of the capacitors are higher, which degrades the efficiency of the power supply, and the current ripple in the output capacitors and the output voltage ripple are greater.